The present invention is directed, in general, to a method of semiconductor wafer fabrication and, more specifically to a method of eliminating agglomerate particles in a polishing slurry used for polishing a semiconductor wafer.
Today""s semiconductor technology is rapidly forcing device sizes below the 0.5 micron level, even to the 0.25 micron size. With device sizes on this order, even higher precision is being demanded of the processes which form and shape the devices and the dielectric layers separating the active devices. In the fabrication of semiconductor components, the various devices are formed in layers upon an underlying substrate typically composed of silicon, germanium, or gallium arsenide. The various discrete devices are interconnected by metal conductor lines to form the desired integrated circuits. The metal conductor lines are further insulated from the next interconnection level by thin films of insulating material deposited by, for example, CVD (Chemical Vapor Deposition) of oxide or application of SOG (Spin On Glass) layers followed by fellow processes. Holes, or vias, formed through the insulating layers provide electrical connectivity between successive conductive interconnection layers. In such microcircuit wiring processes, it is highly desirable that the insulating layers have a smooth surface topography, since it is difficult to lithographically image and pattern layers applied to rough surfaces.
One semiconductor manufacturing process, chemical/mechanical polishing (CMP), is used to provide the necessary smooth semiconductor topographies. CMP can be used for planarizing: (a) insulator surfaces, such as silicon oxide or silicon nitride, deposited by chemical vapor deposition; (b) insulating layers, such as glasses deposited by spin-on and reflow deposition means, over semiconductor devices; or (c) metallic conductor interconnection wiring layers such as tungsten. Semiconductor wafers may also be planarized to: control layer thickness, define vias, remove a hardmask, remove other material layers, etc. Significantly, a given semiconductor wafer may be planarized several times, such as upon completion of each metal layer. For example, following via formation in a dielectric material layer, a metallization layer is blanket deposited and then CMP is used to produce planar metal vias or contacts.
Briefly, the CMP process involves holding and rotating a thin, reasonably flat, semiconductor wafer against a rotating polishing surface. The polishing surface is wetted by a chemical slurry, under controlled chemical, pressure, and temperature conditions. The chemical slurry contains a polishing agent, such as alumina or silica, which is used as the abrasive material. Additionally, the slurry contains selected chemicals which etch or oxidize selected surfaces of the wafer to prepare them for removal by the abrasive. The combination of both a chemical reaction and mechanical removal of the material during polishing, results in superior planarization of the polished surface. In this process it is important to remove a sufficient amount of material to provide a smooth surface, without removing an excessive amount of underlying materials. Accurate material removal is particularly important in today""s submicron technologies where the layers between device and metal levels are constantly getting thinner.
One problem area associated with chemical/mechanical polishing is in the area of slurry consistency. The polishing slurry is a suspension of a mechanical abrasive in a liquid chemical agent. The mechanical abrasive, typically alumina or amorphous silica, is chosen having a design particle size specifically to abrade the intended material. The desired particle size is chosen in much the same way that a sandpaper grade is chosen to give a particular smoothness of finish on wood, metal, or paint. If the particle size is too small, the polishing process will proceed too slowly or not at all. However, if the particle size is too large, desirable semiconductor features may be significantly damaged by scratching or unpredictable removal rates. Unfortunately, because the slurry is a suspension rather than a solution, methods such as continual flow or high speed impellers must be used to try to maintain a uniform suspension distribution. The slurry particles tend to form relatively large clumps when compared to semiconductor device sizes. While these clumps of abrasive can grow to significant size, e.g., 0.1 xcexcm to 30 xcexcm, depending in part upon their initial abrasive particle size, they retain their ability to abrade the semiconductor wafer surface. The agglomeration problem is most apparent when the slurry is allowed to stand. If the slurry is allowed to stand in the supply line for any appreciable time, the agglomeration begins and the slurry can even gel, causing clogs in the supply line or unpredictable removal rates. This results in the need to stop the processing and flush the supply line. Of course, once the supply line is flushed, the stabilized slurry must be reflowed through the line, forcing any residual water from the line. This entire process is time consuming and ultimately very expensive when the high cost of the wasted slurry and the lost processing time is considered. Agglomeration is especially a problem in metal planarization slurries.
To help alleviate this agglomeration problem, the conventional approach has been to keep the slurry flowing in a loop and to perform a coarse filter of the slurry while it is in the loop. To supply the slurry to the polishing platen, the loop is tapped, and the slurry is subjected to a point-of-use, final filter just before it is applied to the polishing platen. However, as the final filter strains out the larger particles, the filter becomes clogged, raising the flow pressure required and necessitating a filter change or cleaning operation. The increased pressure may deprive the polishing platen of slurry and endanger the planarization process. Cleaning or changing the filter clearly interrupts the CMP processing. Naturally, cleaning or replacing the filter is both time consuming and costly. Further, as the filters are extremely fine (capable of passing particles less than about 10 xcexcm to 14 xcexcm in size), the filters themselves represent a significant cost. Additionally, when the processing is stopped to clean/replace the filter, the slurry supply line must be flushed with water to prevent even more agglomerate from forming. This flushing water initially dilutes the slurry when processing resumes, further delaying the CMP process and affecting processing parameters. Unfortunately, even when the filters are flushed regularly, the filters may only last for a period of a few days or even hours, depending upon the daily processing schedule. Furthermore, these filters still allow particles that have particle sizes larger than the intended design particle size to reach the polishing surface.
Another problem area associated with chemical/mechanical polishing is in the area of slurry agglomeration in the slurry drain system. Unfortunately, the abrasive particles in the waste slurry tend to agglomerate also in the drain, forming relatively large clumps. This is a result of the slurry being gravity drained to a waste slurry receptacle at room temperature whereas unused slurry is held at a controlled temperature above room temperature and pumped. The lower room temperature contributes to the waste slurry agglomeration tendency, and the larger agglomerated particles tend to collect in couplings, bends, and internally rough areas of the drain. The agglomeration problem is very apparent if the slurry is allowed to stand in the drain for any appreciable time. When this happens, the drain line may clog. This may require that the processing be stopped and the drain line be flushed. This entire process is time consuming and ultimately very expensive in lost processing time. Agglomeration is especially a problem in metal planarization slurries.
To help alleviate this agglomeration problem in drains, the conventional approach has been to use larger inside diameter drains and to avoid or limit the number of sharp bends in the drain line. Of course, this approach is limited by space constraints in the clean room and does not directly address the problem.
Accordingly, what is needed in the art is a slurry transport system and method of use thereof which efficiently breaks up the CMP slurry agglomerate, and returns the slurry particulate matter substantially to the design particle size.
To address the above-discussed deficiencies of the prior art, the present invention, in one embodiment, provides a method for eliminating agglomerate particles in a polishing slurry. In this particular embodiment, the method is directed to reducing agglomeration of slurry particles within a waste slurry passing through a slurry system drain. The method comprises conveying the waste slurry to the drain, wherein the waste slurry may form an agglomerate having an agglomerate particle size. The method further comprises subjecting the waste slurry to energy emanating from an energy source. The energy source thereby transfers energy to the waste slurry to substantially reduce the agglomerate particle size. Substantially reduce means that the agglomerate is size is reduced such that the waste slurry is free to flow through the drain.
In a particularly advantageous embodiment, the method further comprises sensing a absorbance of the waste slurry with a absorbance sensor coupled to the drain. The method, in another embodiment, includes cycling off the energy source when the absorbance sensed is a nominal absorbance or less. The method further includes cycling the energy source on when the absorbance sensed is greater than the nominal absorbance. In a further aspect, the nominal absorbance may be less than about 0.5.
In one embodiment, the energy transferred to the waste slurry is heat energy. In one specific aspect of this embodiment, the heat energy is transferred with a heating coil. In an alternative embodiment, the heat energy is transferred with hot water. Transferring heat energy with hot water may include injecting hot water or through conduction. In another embodiment, the energy may be transferred as ultrasonic energy by an ultrasonic wave.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those who are skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those who are skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those who are skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.